The present application is a US national application of PCT/IL99/00178, filed Mar. 28, 1999.
The present invention is related to the field of printing and in particular to providing toners with non-standard colors.
Most printing systems are based on actual printing of only a limited number of different standard colored inks. In general, these inks are chosen such that a wide apparent range of colors may be achieved by so called xe2x80x9chalf tonexe2x80x9d processes. In half tone printing, the color is achieved by overlaying partial printed areas of the various standard colors. The percentage of the respective areas printed by the standard colors is chosen such that, to the eye of viewer, the desired color is achieved. In general, cyan, magenta and yellow, optionally together with black are the standard colors, with some systems adding additional colors, such as one or more of green, orange and purple.
It is known to mix colored inks to achieve similar effects. This is sometimes desirable when large areas are to be printed in a given color, since the quality of printing with a single ink is generally better than with half-tones. A custom ink may be mixed from the standard inks by (1) determining the proportions of the standard inks required to achieve the color; (2) determining the thickness of ink in an area printed with the proportions of the colors; (3) comparing the thickness with a desired thickness; and (4) mixing the proportions of the standard colors and a transparent ink and/or adjusting the printing to achieve the desired color, preferably with the desired thickness.
Standard systems have been developed by which a wide range of colors can be achieved by mixing standard colors. Some of these systems are based on color charts. Others are based on computer programs that define the percentages of different standard colored inks (including black and transparent) that should be mixed to achieve a desired color.
It is also known that an approximation of the desired color can be achieved, in a half tone printing process, utilizing only two colors and black. In some systems five or six colors (plus black) are provided. A desired color is provided by half toning utilizing only the two nearest standard colors which form a segment in L*H*C* space and black. This two color process is described for example in co-pending PCT patent application PCT/IL98/00101, filed Mar. 2, 1998 and entitled xe2x80x9cExpanded Color Spacexe2x80x9d, the disclosure of which is incorporated herein by reference. In general such printing results in colors which match the desired color only in a particular lighting situation and deviate from it somewhat in other lighting.
When toners are used for printing, mixing colors is generally impractical. In particular, if toner particles of various colors are mixed together and used for printing, the rate of depletion of the different colors will be different, probably at least because the mobility of the particles is somewhat different. Thus, with use, the color of the toner (and thus of the printed surface) will change as the uneven depletion of the different components of the mix takes place.
One aspect of some preferred embodiments of the invention is concerned with mixing toners of different colors to achieve a mixture which stably prints a target color despite uneven depletion of the components of the mixture.
In a preferred embodiment of the invention, according to this aspect, only two colors of toner particles, plus black and transparent are used to form the mixture. Most preferably, the two colors are colors that are nearby the target color. It has been found that the color achieved when printing with such mixtures is more stable than when a greater number of color components or non nearby colors are used. While it is more desirable to use a greater number of colors in the mixture to achieve better color fidelity, acceptable fidelity, at least under a given lighting condition, can be achieved with only two colors.
In a preferred embodiment of the invention, according to this aspect, color matching is made to a color which is the combination of the color spectra of the actual target color and that of a desired viewing illumination. In a preferred embodiment of the invention, the desired illumination is chosen from a plurality of different illuminations, by an operator and a spectrum of the target color, as viewed in the desired illumination, is computed. This specimen defines what will be referred to herein as an xe2x80x9cillumination adjusted color.xe2x80x9d The two colors are chosen from a group of standard colors.
These two colors are preferably those which form a segment of minimum size in an HC space, in which segment the target illumination adjusted color is contained. Such colors are referred to herein as xe2x80x9cnearby colors.xe2x80x9d The proportions of the two colors and black and the thickness of printed required to achieve a best match to the illumination adjusted color are computed. To the extent that this thickness is less than the standard thickness or range of thickness achieved by a printer on which the ink will be used, a proportion of transparent ink is computed, such that the printed color (with the requisite thickness) will have the desired hue and saturation.
It should be noted that it is believed that this aspect of the invention is also applicable to mixing utilizing more than two toner colors.
In preferred embodiments of the invention, additional factors are taken into account in determining the color mix. These may include, the coloration of the paper or other substrate on which the color is to be printed, the difference between the proportions of toner in mixture and on the developed image and the difference between the proportions of toner left on the developed paper after metering of the developed image.
There is thus provided, in accordance with a preferred embodiment of the invention a method of determining proportions of toner particles of different colors to be used in a mixture for toning an electrostatic image, the method comprising:
determining a target color for matching;
determining at least one set of candidate colored toner particles;
computing a mixture of toner particles of the at least one set of candidate color toner particles that best matches target color, and
mixing the colored toner particles to form a toner for printing.
In a preferred embodiment of the invention, determining a target color comprises:
determining a target color,
determining an illumination under which the color is to be viewed;
determining the target color for matching as the apparent color of the target color under the illumination.
Alternatively, determining a target color comprises:
determining a target color;
determining an illumination under which the color is to be viewed;
determining a color of a substrate on which the target color is to be printed; and
determining the target color for matching as the apparent color of the target color, printed on the substrate, under the illumination.
Preferably, determining the color of a substrate comprises choosing a substrate type from a plurality of substrate types.
In a preferred embodiment of the invention, determining an illumination comprises choosing an illumination from a plurality of illumination types.
In a preferred embodiment of the invention, determining at least one set of candidate color particles comprises choosing two types of particles both of which are different from black and transparent. Preferably, determining the at least one set of candidate color particles comprises choosing said particles from a plurality of available color toner particles.
Preferably,determining the at least one set of candidate color particles comprises choosing the pair of colored toner particles in said plurality of available color toner particles, which pair comprises colors which are closest to the target color.
In an alternative preferred embodiment of the invention, determining the at least one set of candidate color particles comprises choosing a plurality of pairs of said particles. In a preferred embodiment of the invention determining the at least one set of candidate color toner particles comprises choosing all pairs of available color particles.
In a preferred embodiment of the invention, the method includes:
determining a best match proportionate mixture for each set of color pairs which best matches the target color; and
choosing the color pair and proportions having the best match to the target color.
Preferably, when more than one color pair have the same best match, the pair having the closest colors is chosen.
In a preferred embodiment of the invention, computing a mixture of toner particles comprises comparing the target color to a calculated printed color of the color toner mixture under the conditions under which it is to be viewed.
Preferably, the method includes:
correcting the mixture of toner particles to be mixed, responsive to the relative transfer proportions of the toner particles to the electrostatic image during development of the image.
In a preferred embodiment of the invention the method includes mixing black and transparent toner particles with the colored toner particles to form a better color match between the target and a color printed with the mixture.
In a preferred embodiment of the invention, the toner particles are provided in separate liquid toner dispersions of toner particles in carrier liquid and mixing the toner particles comprises mixing the liquid toner dispersions.
In a preferred embodiment of the invention the method includes correcting the mixture of toner particles to be mixed, responsive to the proportion of toner particles of different types removed from a developed image during processing of the image.
There is further provided, in accordance with a preferred embodiment of the invention, a toner comprising a mixture of toner particles produced according to the invention.
There is further provided, in accordance with a preferred embodiment of the invention, a method of printing comprising:
providing an electrostatic printing apparatus; and
printing images in the apparatus utilizing toner according to the invention.
In a preferred embodiment of the invention, the method includes adjusting the thickness of the printed image in order to provide a better match to the target color.